The present invention relates generally to alternating current (AC) motors and, more particularly, to a system and method for optimizing current filtering in a motor soft starter.
Motor soft starters are devices that control transmission of voltage and current from an AC power source to an induction motor. Soft starters are configured to limit the transient voltages and current to the induction motor 40 during start-up, resulting in a “soft” motor starting. In operation, power from the AC source is passed through switching devices in the soft starter, such as a pair of anti-parallel solid-state switches in the form of silicon controlled rectifiers (SCRs), to control the current flow and, in turn, the terminal voltages of the induction motor.
It is recognized that current measurements are a key to soft starter control and operation, and are the basis for many control and protection algorithms. It is also recognized that filtering is performed on the current measurements as a way to attenuate noise and more accurately represent the true system current characteristics. However, the level of filtering that occurs must be balanced with the dynamic requirements of the soft starter and its ability to respond quickly and appropriately.
The noise present on soft starter current measurements is typically higher in frequency than the actual current signal itself. As such, low pass filtering is performed to attenuate higher frequency content. However, the greater the reduction in the higher frequency content, the slower the current measurement will reflect actual changes. Thus the soft starter operation is characterized according to how often current is expected to change and how quickly those changes need to seen by the internal control. Soft starter current characteristics change depending on what mode of operation the soft starter is in. For ramp-up/start-up scenarios, the current is changing quickly as the system responds to the motor start dynamics, and the soft starter operates in a start-up mode using the SCRs as switching devices. After the motor has started, changes in current are less frequent, and the soft starter operates in a bypass mode such that AC power is transferred to the induction motor via a contactor that bypasses the SCRs.
Existing technology does not differentiate between soft starter operational modes for current filtering, nor employ an associated adjustment to the filter parameters. The current filtering is thus static across each mode of operation.
It would therefore be desirable to design current filtering for the soft starter that can take into account these different expected conditions, and optimize the current filtering accordingly. It would also be desirable for such current filtering to optimize noise removal and more quickly correlate the current measurement to actual changes.